1. Description of the prior art
There is a great deal of interest in the development of bipolar switching semiconductor devices. Prior art structures include the IGBT, MCT, and the DMT. In typical switching devices it is preferable to conduct the on state current via a thyristor structure because this mode of operation results in lower forward voltage drop and reduced power dissipation. MOS gated control turn-off is desirable because the high input impedance simplifies the drive circuit and reduces cost. These features are achieved in the MCT by using a MOSFET within the base region of the thyristor to create a substantially short circuit between the emitter and base of the thyristor. In the case of the DMT a depletion mode MOSFET is used to pinch off the base current within the thyristor to achieve turn off.
2. Summary of the invention
The switching device which is the subject of this invention can be conveniently considered as a typical thyristor in combination with a MOSFET device used to decrease the base resistance and the base current when the thyristor is in the on-state. Reducing the base current increases the holding current. Since the holding current of the thyristor is inversely proportional to the base resistance, raising the holding current to above the operating current by reducing the effective resistance of the base region, results in turn off of the thyristor. Although this can be done in several ways the preferred method is to use a MOS gate structure as disclosed in this Patent Application.
DEFINITION: For purposes of this patent application the term "effective base resistance of the base region" means the electrical resistance between the upper base region and the cathode electrode along all paths by-passing the PN junction between the upper base region and the cathode region.
More specifically, the preferred embodiment of the invention comprises a four layer semiconductor switching device which includes: a semiconductor structure comprising an anode region, a lower base region, an upper base region, a cathode region, and a diverter region; an anode electrode affixed to the anode region; a cathode electrode affixed to said cathode region; and gate means disposed on and insulated from one surface of the semiconductor structure for modifying the effective resistance of at least one of the base regions thereby diverting current to the diverter region to reduce the internal feedback of the switching device sufficiently to turn off the semiconductor switching device.